Conventionally, an electrophotographic printing image forming apparatus, such as a copying machine, a printer, or a facsimile, has been known. According to such an electrophotographic printing image forming apparatus, an electrostatic latent image is formed on a surface of a photoreceptor (e.g., photoreceptor drum), and toner is supplied to the photoreceptor drum by a developing device so that the electrostatic latent image is developed. After that, the toner image formed by the developing on the photoreceptor drum is (i) transferred onto a sheet such as paper and then (ii) fixed on the sheet by a fixing device.
In recent years, an image forming apparatus, which deals with full color images with high image quality, uses two-component developer (hereinafter, simply referred to as “developer”) because of excellence in stabile electrostatic property of toner.
Such a developer contains toner and carrier, which are stirred in a developing device so as to be rubbed together, and such friction provides properly charged toner.
The toner charged in the developing device is supplied to a surface of a two-component developer holding member such as a developing roller. The toner supplied to the developing roller is moved, by electrostatic attractive force, to an electrostatic latent image formed on the photoreceptor drum. Consequently, a toner image is formed on the photoreceptor drum in accordance with the electrostatic latent image.
Recently, an image forming apparatus has been demanded to operate at a higher speed and to be reduced in size. In order to meet such demands, it is becoming necessary (i) to charge the developer quickly and sufficiently and also (ii) to carry the developer quickly.
Under the circumstances, some image forming apparatuses are configured to include a circulating developing device in which supplied toner is immediately dispersed in developer so that an appropriate amount of charge can be obtained.
Such a circulating developing device includes (i) a developer carrying path through which developer is carried to circulate, (ii) a screw auger (developer carrying member) which stirs and carries the developer in the developer carrying path, (iii) a toner supply inlet through which toner is supplied from a toner storing section into the developer carrying path, and (iv) a toner density detecting sensor which detects a toner density of the developer. In the circulating developing device, in a case where the toner density of the developer becomes lower than a predetermined value, toner is supplied to the developer carrying path in response to an instruction, to a cartridge, on supplying a toner, and the supplied tone is to be carried while the developer is stirred (see Patent Literature 1).
Moreover, a developing device has been proposed in which, in order to maintain stable toner density by supplying toner, toner is supplied from a toner storing section into a developer carrying path based on an amount of toner, which is to be consumed in forming an image, predicted by the use of a dot-counting device. In the developing device, toner empty (toner shortage), in which toner to be supplied does not remain in the toner storing section, is detected by a toner density sensor which is made up of a magnetic permeability sensor and is provided near a toner supply inlet (see Patent Literature 2).